[doc] Add all the legacy specifications to the monorepo

Summary: The files are moved as-is from their current location at https://github.com/bitcoincashorg/bitcoincash.org/tree/master/spec. They might not be up-to-date and need some rebranding but this is a first step.

Test Plan: Check all the docs are here

Reviewers: #bitcoin_abc, Mengerian

Reviewed By: #bitcoin_abc, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D17597